3,5-Diaminobenzotrifluoride is useful as a monomer for the production of polyimide resins. As produced, the product often occurs with several percent of 4-chloro-3,5-diaminobenzotrifluoride as an impurity. The diamine and the chloro-diamine have similar boiling points and are, therefore, difficult to separate by distillation. They have similar crystallization properties and are, therefore, difficult to separate by recrystallization.
Canadian Patent 1,270,855 discloses a process for separating a para-fluoroaniline from the corresponding non-fluorine substituted aniline. Thus, for example, para-fluoroaniline can be separated from aniline and para-fluoro-meta-chloroaniline can be separated from meta-chloroaniline. The process involves dissolving the anilines in an aqueous solution of a non-oxidizing acid such as hydrochloric acid, and fractionally crystallizing the hydrochloride salts. The para-fluoroaniline crystallizes in preference to the non-fluorinated aniline, and accordingly, the crystalline product produced is enriched in the para-fluoro compound. Depending on the degree of purity desired, several recrystallizations may be required. Further quantities of the para-fluoroaniline can be recovered from the mother liquor, since upon evaporation, the para-fluoro aniline will crystallize in preference to the unfluorinated aniline.
U.S. Pat. No. 3,452,096 discloses a method for separating 2,3-dichloroaniline from 2,5-dichloroaniline and 3,4-dichloroaniline. The method involves treating a mixture of dichloroaniline with either a monovalent inorganic acid or with an aroylhalide to produce either the acid addition salt of 2,3-dichloroaniline or the n-acyl derivative of 2,3-dichloroaniline. The acid salt or the n-acyl derivatives of 2,3-dichloroaniline are less soluble than those of the other isomers and, therefore, precipitate preferentially.
U.S. Pat. No. 4,532,352 discloses a process for the separation of para-fluoroaniline from a mixture containing aniline and para-fluoroaniline. The process comprises contacting the mixture with a mineral acid in the presence of an organic solvent and separating the insoluble hydrochloride salt of para-fluoroaniline by conventional means such as filtration. If desired, the isolated salt can be converted to para-fluoroaniline by reaction with a base.